Did it really take until 1990 for "Europe"/UK to switch from glass bottles of milk to cartons?

Not sure how the situation in the UK should have much relevance for the rest of a whole continent without (at that time) free trade, but anyway: Both East and West Germany had milk in plastic bags from the 1960s or 1970s on. Milk in (tetraedrical) tetra paks was available in East Germany since at least the 1970s. On the other hand super markets in Germany even today usually offer some of their milk (the relatively untreated and more expensive variety) in glass bottles.

If you read the wiki article about Tetra Pak, you will find out that tetra paks were a) invented in Europe in the 1950s and b) used for milk and milk products from the very beginning. So the answer to the question is "definitely not".

Q: Did it really take until 1990 for "Europe"/UK to switch from glass bottles of milk to cartons?

No. This is all based on flawed premises.

Additionally, this is nowhere about anything 'Europe', but simply Wikipedia misleading its readers with taking the situation in the UK as a pars pro toto for Europe. Which would be bad enough on its own, but made much worse by mixups it contains.

The bit quoted from Wikipedia is misquoting its sources:

Q: By 1975, 94% of milk was in glass bottles, but in 1990, supermarkets started offering plastic and carton containers, reducing bottled milk from 94% to 3% by 2016.

But the source says nothing about "started in 1990", says nothing about 'carton' or 'TetraPak', at all, but instead gives this account about the 'comeback of the glass bottle':

Yet by the 1990s major supermarkets had switched to cheap plastic bottles, buying in bulk to drive down prices, and leaving the milkie struggling for business. […]

In 1975, 94 per cent of milk was put into glass bottles, but that had fallen to under three per cent by 2016. However despite ongoing slumps in sales of cow milk, as people switch to nut, rice and soya options, the market for glass bottles milk appears to be holding steady, and even slightly increasing.

— Sarah Knapton: "Milk floats and glass bottles make a comeback as shoppers shun plastic", The Telegraph, 21 January 2018.

Thus, before asking about anything like the implications found in the question, we need much better statistics to base our inquiry on, better than this unreliable snippet from Wikipedia.

Since Wikipedia says nothing really about 'Europe', but solely about market shares of containers for milk in the UK, with the source for that horrible snippet being seemingly more precise, but still very wobbly, a better statistic might inform us with:

Table: Shares of UK liquid milk market by container type (%), Source: EU 2005

— Paul Vaughan, Matthew Cook & Paul Trawick: "A Sociology of Reuse: Deconstructing the Milk Bottle", Sociologia Ruralis, Vol 47, No 2, April 2007. pp.120–134, doi

Or in an updated graphic:

— Sarah Greenwood et al.: "Buy the product but rent the packaging – making reusable plastic packaging mainstream", in: UK Plastics Circular Network (ed): "Plastics Research and Innovation Fund Conference—Creative Circular Economy Approaches to Eliminate Plastic Waste", 2020, pp.26–37. pdf

So the start of TetraPak-like cartons was even in the UK not really only 'started in the 1990s'?

More important: the UK is known for coming up with it and keeping it going for much longer than elsewhere, especially 'in Europe': fresh milk delivery to the front door. The above table shows total market share for consumer milk containers. Which does include pasteurised and long-shelf-life products. And in the UK for quite a while the milk was still delivered fresh to the home in substantial proportions, not so much store-bought as it was in most of the rest of Europe much earlier. Store-bought milk in the UK saw 'glass-alternatives' earlier than the Wikipedia-quote suggests. UK's milk delivery kept glass usage for that milk system much higher than elsewhere.

A note on environmental effects for different container types: here glass bottles. Most recent studies modelling the effects of heavy to transport glass containers are assuming a reuse-rate of 8. And conclude in their models: plastic is better for carbon emissions. The above makes its model run the consumer milk bottle to be re-used 280 times! Which, I am sure it is mere coincidence, shows that glass is by far the most eco-friendly transport vessel. ;)
As with all models, and especially the mathematical ones giving you an illusion of precision, re-sing that Kraftwerk song about a model: 'and she's looking good'…

So, in light of having a nice system in place, very convenient and with perfectly suitable for the task materials, perhaps the question should be re-phrased to: 'why did they UK start to switch away from the milkman delivery system with glass bottles, at all, in these proportions, let alone with this specific timeline?'

The most important UK-specific aspects are summarised as:

Barriers to Reuse

Throughout the food and drink industry, the introduction of plastic as a packaging material has brought new capabilities. Plastics ‘plasticity’ has enabled milk bottles to be shaped with a handle making it easier to carry and pour. The ability to create bottles of a larger size coupled with developments in domestic refrigeration has allowed milk to be sold in bulk and stored for longer in the home. Purchasing milk in higher volumes reflects a wider social trend of bulk shopping instead of daily grocery shopping as the supermarket superseded the milkman. In 2016, almost two-thirds of milk purchases were in a four pint or two-litre container, whilst only 4.9% were bought in the traditional doorstep pint size.

A report on refillable glass container systems reported that “the traditional one pint refillable glass milk bottle [was] not suited for the retail shelves”. Indeed, whilst glass was a valued material for packaging products, it also had its critics. Marks and Spencer’s Packaging Department offers an insight of the emergence of plastics (specifically PET) and its relationship with glass. Compared to plastics, glass packaging contributed considerably to the overall weight of the product, thus reducing the overall potential spend in store. Glass was also prone to breakage and doorstep milk bottles were susceptible to slipping through the fingers. Plastic on the other hand, if dropped, would not smash or cause damage in the home.

The return system of glass bottles was also a victim of social and cultural changes. With the increase of women in the workplace, milk was often left out on the front porch all day if delivered too late. Between 1993 and 1996, liquid milk consumption fell by 14%, which Ward attributes to traditional products such as milky drinks, rice pudding and custard being replaced by pre-prepared convenience foods. Dietary issues related to animal fats also changed the public perception of milk resulting in the more diverse milk market that we see today.

During a sitting on the ‘Milk-based Drinks Regulations’ (1983), doorstep delivery was highlighted as being paramount for the elderly, the mother and her young children and those without easy access to supermarkets. Whilst doorstep delivery was a highly effective means of selling goods then, when car ownership and public transport infrastructure were limited, people now generally have better access to supermarkets.

Market factors, particularly the deregulation of the Milk Marketing Board (MMB) in 1994, accelerated the demise of ‘the daily pinta’. The MMB’s main activity was the collection and sale of milk in England and Wales. The 1980s and 1990s were a time of radical change within the milk market with the Conservative Government’s privatisation and the Board’s incompatibility with the EU regime. During this period, large producers sought to sell to the supermarket groups directly, avoiding the scheme entirely. With its deregulation in 1994, its successor ‘Milk Marque’, a voluntary cooperative, controlled 65% of the milk in England and Wales. Criticised as an unregulated farmer-owned monopoly, Andrew Dare, chief executive of Milk Marque argued that the non-profit-making co-operative provided a vital service in striving to pay dairy farmers a fair return. In the summer of 1995 a pint of supermarket milk sold at 28p, whilst a four-pint plastic bottle cost only 22p a pint. In comparison, doorstep delivery milk sold for between 36p and 41p per pint. Referring to this disparity, Dixon highlights that supermarkets were able to reduce the price of milk for the consumer by removing the cost of the delivery service.

Leading dairies reacted to and arguably accelerated the transition of milk sold in plastic bottles in supermarkets. For example, Northern Foods restructured their business reducing their glass-bottling capacity and contracting the doorstep market whilst continuing to invest in the supermarket sector. Developments in the milk industry supported the centralisation of bottling and the operation of scale economies. Major capital investment focused on product and packaging innovation in developing close relationships with major multiple retailers. Refrigerated storage and transport improvement throughout the supply chain increased the shelf life of milk and enabled plastic bottles to be rolled onto lorries, into supermarkets and then directly into supermarket aisles.

Enablers of Reuse

Today, the majority of milk is sold in plastic bottles from the supermarket, however bottled milk in glass has retained or acquired positive associations in terms of provenance, citizenship, sustainability and convenience.

Today's doorstep Milk Deliverers argue that their milk is fresher, superior in taste and has provenance compared to supermarket milk. Tracing this back, the doorstep pint has regularly been associated with a rural idyll, coming from local farms via a regional diary whilst milk from supermarkets is associated with a larger nationalised distribution scale. The glass milk bottle then communicates traceability and some consumers are happy to pay more in support of British farmers. Vaughan et al. also highlight that consumers continue with the milkman service as a way of keeping him employed and to provide social functions such as keeping an eye on the elderly.

The glass milk bottle is generally regarded as one of the most environmentally friendly forms of packaging, which became ever more important with the emergence of its plastic competitor. Many environmental groups saw the transition from reusable glass to plastic as a retrograde step. Most recently, the ‘Blue Planet Effect’ has contributed to customers moving back to the glass bottle.

— Greendwood; amply referenced and really hitting most points mentioned in many comments on this page…

At this point, the 94% of UK milk sales in glass bottles was not sold in shops, but rather delivered via a local subscription scheme using fully recyclable containers delivered by electric vehicles. Honestly, add an app and it'd sound like a startup pitch from a couple of years ago.

Glass bottles are indeed relatively heavy and awkward - but this is only a problem if you have to carry them home from the shops. Once they're at your door, the disadvantages are much less. If you drop it in your kitchen, you're having a bad day whether it's in smashed glass or broken-open card.

The van collected empties as it delivered fresh ones. Glass could be reused extensively, and the net cost of cleaning and refilling was less than filling card containers. It worked out well - the shift away from it was as much due to changing shopping habits as anything else.